1) Field of the Invention
This invention relates generally to fabrication and structures of semiconductor devices and more particularly to the fabrication of an Electro Static Discharge (ESD) device and more particularly an Electro Static Discharge (ESD) device using a silicide process. The invention related to a device for on-chip ESD protection.
2) Description of the Prior Art
The nxe2x88x92 type MOS transistor has been widely employed as the primary component for an ESD protection circuit in semiconductor IC devices. It is well known that silicidation of the drain and LDD junctions reduce ESD performance significantly. Most salicided process have a removal option which allows unsalicided areas (e.g., resistors) to be formed and use ESD implant to make junction deeper and to overdose the lightly doped region of the LDD for better ESD performance.
NMOS transistors stacked in a cascade configuration provide robust ESD protection for mixed voltage I/O in both silicided and silicide-blocked technologies. However, this kind of device has high snapback voltage. Also, the high snapback voltage of the stacked NMOS degrades its IT2 (IT2 is the second breakdown trigger cuffent)) since the power dissipation is great. The IT2 is the current at or before the MOS gets into secondary breakdown (thermal/permanent damages) The higher the It2, the more robust the NMOS and the higher the ESD threshold. For the process technology where the silicide block and abrupt junction steps were are not available, a biasing network was necessary to ensure uniform triggering of all fingers. So, the need for high voltage tolerant I/O""s severely complicates ESD protection.
FIG. 5A shows a single poly Nxe2x88x92 MOS device that is used in the prior art as an ESD device. The structure and snap back mechanism are described below. The single-poly N-MOS device is shown in cross section and layout in FIG. 5A. FIG. 5B shows a top plan view. FIG. 5C shows the IV curve and snap back curve for the ESD device. Vsp is the snapback holding voltage. FIG. 5D shows the electrical schematic of the device in FIG. 5A. When a short-duration (100 to 110 ns) constant current pulse is applied to the drain with the source and gate tied to the substrate (substrate grounded), the device should have the I-V characteristic shown in FIG. 5C. At normal operation, the device is off because the gate is grounded. When the drain breakdown voltage, BVdss is reached, current starts to flow as a result of impact ionization of die reverse-biased drain junction. At current It1, and voltage Vt1, the device triggers into snapback. The trigger current It1 and voltage is related to the channel length and BVdss. Note that the trigger voltage point (Vt1, It1,) is not the same as BVdss. BVdss, usually is defined as the drain junction avalanche breakdown voltage at a specified drain current density. The trigger point is the point that has the highest voltage just before snapback. The snapback region of the I-V curve is roughly linear and, therefore, may be represented by a snapback voltage Vsb and a differential resistance Rsb. The snapback voltage Vsb is defined as the linear extrapolation of the snapback region back to zero current. Care must be taken to avoid defining Vb and Rb by extrapolating from low current values near the point where the I-V curve changes slope from negative to positive. Therefore, the values of Vsb and Rsb were obtained from measurements made at high currents with the transmission-line pulse technique. Because the high-current values are relevant to ESD events, we need to use them rather than the low-current values when designing for protection against ESD. With sufficiently high current It2, flowing in the snapback region, the device triggers into second breakdown. We define a second trigger point (Vt2, It2) corresponding to the triggering from snapback into second breakdown. Second breakdown is the term used for power bipolar devices to indicate the regime of thermal runaway and current-instability.
The following patents show related ESD devices: U.S. Pat. No. 5,898,205 (Lee), U.S. Pat. No. 5,519,242 (Avery), U.S. Pat. No. 5,969,923 (Avery), U.S. Pat. No. 5,559,352 (Hsue et al.), U.S. Pat. No. 5,043,782 (Avery) and U.S. Pat. No. 5,689,113 (Li et al.).
There is a challenge to device a new ESD device for silicide process that improve the ESD performance by lowering the Vt1 and lower leakage.
It is an object of the present invention to provide a method for fabricating an ESD device with a lower trigger voltage and lower leakage.
It is an object of the present invention to provide a structure of an Electro Static Discharge (ESD) device that provides better protection without process changes and additional costs.
It is an object of the present invention to provide an IC design having a structure of an Electro Static Discharge (ESD) device to be used in IC chip manufactured with a silicided process.
It is an object of the present invention to provide a method and a structure of an ESD device that overcomes the problems associated with silicided drains.
It is an object of the present invention to provide a method and a structure of an ESD device for a silicided process that does not add any extra process steps or cost.
It is an object of the present invention to provide a method for fabricating a ESD device with silicide contacts with a lower trigger voltage and lower leakage using pseudo parasitic bipolar transistors.
To accomplish the above objectives, the present invention provides an ESD device and method of fabricating such device using parasitic NPN transistors. The invention has four preferred embodiments.
The first embodiment is a parasitic Bipolar Junction Transistor comprised of n+/nxe2x88x92/pxe2x88x92/nxe2x88x92/n+ regions. The emitter is formed of the N+ region and the second Nxe2x88x92 well. The parasitic base is formed by the pxe2x88x92 substrate or well. The collector is formed of the first well and the first n+ region.
The benefit of the first embodiment is the trigger voltage is lower because the junction between the nxe2x88x92 well (emitter) and Pxe2x88x92 substrate (base) and the junction between Pxe2x88x92 substrate (base) and the nxe2x88x92 well have lower cross over concentrations. The lower the cross over concentration, the lower the trigger voltage (Vt1, It1). The invention""s cross over concentrations are lower than conventional NPN devices that use the N+, P junctions and do not have the invention""s first and second nxe2x88x92 wells.
The second embodiment is similar to the first embodiment with the addition of the first gate. The first gate is preferably connected to the first n+ region and the Vpad (connected to Vdd).
The third embodiment contains the same elements as the second embodiment with the addition of a third n+ region. The third n+ region is preferably shorted (or connected) to the first p+ region and the second n+ region. The third embodiment forms a second NPN parasitic bipolar using the third N+ region as an emitter.
The forth embodiment contains the same elements as the third embodiment with the addition of a second gate over the first isolation region. The second gate is preferably connected to the third n+ region to the first p+ region and the second n+ region. The gate changes the electrical characteristics of the first parasitic bipolar transistor.
Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of instrumentalities and combinations particularly pointed out in the append claims.